1. Field of the Invention
The present invention relates to watermakers that produce fresh water from sea water, and in particular to reverse osmosis types of water desalination systems that use electrostatic water treatment to control membrane fouling.
2. Description of the Prior Art
Water filtration devices and the benefits of filtered water are widely known to the public. Filtration can span very coarse devices that merely strain out solids, to finer filters that can remove fine colloids and particulates. At the extreme end of the filtration spectrum is reverse osmosis (RO). Salt, or otherwise contaminated, water forced through an RO membrane will produce clear, fresh water permeate. Such highly purified water finds uses in semiconductor fabrication, microelectronics manufacturing, pure water beverages, pharmaceuticals, drugs, and other health products. In particular, RO can be used to desalinate seawater.
Boats can only carry limited amounts of fresh water in their holding tanks, so long voyages can be constrained by how much fresh water can be carried before having to replenish. Reverse osmosis (RO) water filtration systems have become commercially available that can convert sea water into fresh water. For example, on the Internet see, www.spectrawatermakers.com/
Prior art watermakers for boats have required special chemicals to clean and store membranes during periods of lay-up and hibernation. Such maintenance is expensive, inconvenient, and time consuming. A long period of non-use may not be planned, and the lack of preparation can precipitate problems later with fouling.
Advanced RO systems now primarily use spiral wound polyamide membranes. In a typical two-layer design, a polysulfone membrane supports a very thin skin-layer that is polymerized in-situ. These, however, are especially sensitive to fouling. Some RO systems use water softeners or chemical injection to pre-treat the incoming water and prevent scale that would otherwise foul the RO membranes.
Rodrigo F. V. Romo and M. Michael Pitts describe an electrostatic method to prevent RO membrane fouling in “Application of Electrotechnology for Removal and Prevention of Reverse Osmosis Biofouling,” published in AIChE Environmental Progress, Vol. 18, No.2, Summer 1999, pp. 107–112. Other, earlier work on electrophoresis or Zeta Potential was mentioned as being done by T. M. Riddick in the 1960's. Romo, et al., describe three kinds of RO membrane fouling, e.g., crystalline, particle and colloid, and microbiological. In order for biofouling to occur, the micro-organisms must attach themselves to a surface. Their paper describes using electrostatic charging of the colloids and particles in suspension to prevent aggregation and attachment that would otherwise lead to biofouling.
M. Michael Pitts describes in U.S. Pat. No. 5,591,317, issued Jan. 7, 1997, (Pitts '317), an electrostatic-field generator for water treatment. A high voltage direct current source is capacitively coupled to a volume of water. The majority of the voltage gradients are said to be generated in the water, and charge particles and colloids in the water so they repel one another.
The ZETA-ROD marketed by Zeta Corporation (Tucson, Ariz.) is a commercial implementation of the electrostatic-field generator described by Pitts '317. The prior art places such electrostatic-field generators in pipes with constant or frequent flows of water. So the treated water is constantly immersing the filters downstream.
A problem developed when such electrostatic-field generators were used in RO systems that were shut down or not operated for long periods. The treated water does not reach the RO membranes when the water is stagnant. The RO membranes foul as a result of being left stagnant too long. What is needed is an RO system that uses electrostatic-field generators in such a way that the RO membranes are constantly bathed in treated water.